As you said, you're a user not a programmer. The work involved is complex!
For example, how would you know where to insert the lines of code? The only reason some installers are like that are because;
#1 They're IPS components like the blog/gallery which already have the code in those areas. (not all of it)
#2 They used the components system
#3 They don't require heavy edits.
How would you know where to insert the code? Putting something like insert code into line 55 isn't accurate as then the mod might as well be .x.x specific. Every .x.x release does have an change, and then what about conflicting modifications? What if, you had some instruction to insert under
CODE
//=====
// BLAH!
//=====
$lawl = 2
echo "rawr!"
include ("blah/blah.php")
But then, what if this code was modified by another modification? Then how would they know where to insert it? You can't go by line numbers either since that's probably changed. I don't really see a solution into this unless there were specialiy alloted areas in the code designed for modifications. And not all of the board would have it.